The present invention relates generally to a process for making a compact and low-leakage diode, and more specifically relates to a process for making a polysilicon-based diode having a relatively high ratio between the resistance to forward conduction and the resistance to rearward conduction; i.e., the diode on/off ratio. One exemplary preferred implementation of this diode is in a chalcogenide-based memory array in an integrated circuit.
Chalcogenide materials have recently been proposed to form memory cells in memory devices. As is known to those skilled in the art, a memory device can have a plurality of memory arrays, and each memory array can include hundreds of thousands of memory cells. Each memory cell generally includes a memory element and an access device, such as a diode, coupled to the memory element. The chalcogenide materials store information by changing resistivity. Generally speaking, chalcogenides are materials which may be electrically stimulated to chance states, from an amorphous state to a crystalline state, for example, or to exhibit different resistivities while in the crystalline state. Thus, chalcogenide memory elements can be utilized in memory devices for the storage of binary data, or of data represented in higher based systems. Such memory cells will typically include a cell accessible, for example, by a potential applied to access lines, in a manner as conventionally used in memory devices. Typically, the cell will include the chalcogenide element as a resistive element, and will include an access or isolation device coupled to the chalcogenide element. In one exemplary implementation suitable for use in a random access memory (RAM), the access device will be a diode of the structure disclosed herein.
Many chalcogenide alloys may be contemplated for use with the present invention. For example, alloys of tellurium, antimony and germanium may be particularly desirable, and alloys having from approximately 55-85 percent tellurium and on the order of 15-25 percent germanium are currently contemplated for use in chalcogenide memory cell devices. U.S. Pat. No. 5,335,219 is believed to be generally illustrative of the existing state of the art relative to chalcogenide materials, and is believed to provide explanations regarding the current theory of function and operation of chalcogenide elements and their use in memory cells. The specification of U.S. Pat. No. 5,335,219 to Ovshinski et al., issued Aug. 2, 1994, is incorporated herein by reference, for all purposes. An exemplary specific chalcogenide alloy suitable for use in memory cells in accordance with the present invention is one consisting of Te56Ge22Sb22.
A diode as disclosed herein is of use in many different applications. In the exemplary use of the diode in a chalcogenide memory cell, the attributes of the current device are especially significant. In a chalcogenide memory cell, it is desired that the diode have a lower forward resistance than the lowest possible resistance state of the chalcogenide element. Likewise, a preferred diode would have a hither reverse resistance than the highest resistance state of the chalcogenide elements. Given that chalcogenide elements having a broad range of resistance states are desired, there exists a need for a diode having a very high ratio of forward resistance to reverse resistance (on/off ratio). For example, a ratio on the order of 1,000,000:1 has been discussed as a desired goal.
Polysilicon based diodes have the potential for providing such a ratio. However, traditional polysilicon diodes have exhibited relatively high leakage due to grain boundaries which provide leakage paths. This occurs because current conducts along the grain boundaries. Accordingly, the need remains, for a low-leakage diode having a relatively high ratio of forward resistance to reverse resistance and for a method to manufacture such a diode. The present invention offers a novel polysilicon diode construction, and a method of manufacturing such a diode having an improved high on/off ratio and improved leakage resistance characteristics.
The present invention provides a new diode, which may be manufactured to exhibit improved resistance to leakage; and also encompasses memory cells incorporating such diode and their method of manufacture.
In accordance with the present invention, the diode will be formed in a volume of polysilicon material containing the p-n junction. The diode is constructed to promote current conduction through the diode along a path which is perpendicular to the grain boundaries in the polysilicon. This is accomplished by configuring the polysilicon through control of the deposition parameters to orient the grain boundaries in a predetermined orientation, and by forming the polysilicon to avoid deleterious conductive paths, and to control the direction of current flow through the diode.
In one particularly preferred implementation, the polysilicon material will be formed within a container, such as within a volume of an insulating material. Preferably, the polysilicon material will be formed so as to define a generally central void therein. In one particularly preferred implementation, the polysilicon element will include a first, generally solid portion; and will include a second, generally annular portion extending therefrom. Thus, viewed in vertical cross-section, such an embodiment exhibits a generally U-shaped cross through at least a portion of the polysilicon. By xe2x80x9cannularxe2x80x9d, it is not intended to define that the second portion would be circular in shape, but that there would be an outer perimeter area of polysilicon which would extend around an opening. The opening is provided so as to preclude communication of grain boundaries, formed by the deposition of the polysilicon material, across the width of the container. This void or opening will be filled with a generally insulating material. A junction will be formed within the polysilicon, such as through doping of the polysilicon, in accordance with known techniques.
In one particularly advantageous implementation of the invention, the diode will be used in manufacturing memory devices, including memory cells, with such cells including a chalcogenide multiple resistive state element in electrical communication with the diode. In such a memory cell, the diode serves as the access device, and the improved on/off ratio of the diode as described herein may be used with substantial advantage.